The present invention relates to processes for the separation of a gaseous mixture and in particular to a process for the production of nitrogen rich gas from air.
Processes are known for the separation of nitrogen from air employing an adsorbent material which has the ability to effect separation as between the two major components of air by virtue of its more rapid adsorption of oxygen that nitrogen. The adsorbent is typically molecular sieve carbon, for example, as described in Japanese patent publication (Kokoku) No. 18675/1977.
In operation, a bed of the adsorbent is put through a cycle of operations which includes an adsorbent step during which time air is passed through the bed and most of the oxygen and a proportion of the nitrogen and substantially all the carbon dioxide and water vapour in the feed are adsorbed, and a nitrogen-rich product gas is supplied from the outlet of the bed; and a desorption step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and/or evacuated through its inlet so that the adsorbed gases are substantially removed from the bed, thereby preparing it for the next adsorption step.
In practice, two adsorbent beds are employed and operate on similar cycles which are sequenced to be out of phase with one another by 180.degree. so that when one bed is on during its adsorption step, the other bed is on its desorption step and vice-versa.
However, although the known nitrogen producing systems are satisfactory for the the supply of product gas on a continuous basis, should they have to be closed down, then after a fresh start-up a considerable time is required before the product gas reaches an acceptable purity. Normally, the process is halted (closed down) during a pressure equalisation step to make a fresh start-up easier. However, if the apparatus is allowed to stand for a long time, it has been found that oxygen which was originally adsorbed by the molecular sieve carbon is desorbed and on start-up the oxygen dilutes the nitrogen product gas. As a result, the time after start-up before an acceptable purity of product gas can be supplied is prolonged. This can necessitate the discharge of enriched nitrogen of insufficient purity from the apparatus prior to an acceptable level of purity being obtained. Thus, when the known nitrogen producing systems are used on an intermittent basis, their operating efficiency is low.